1. Field of the Invention
The present invention relates to an image pickup apparatus that does not require a focus-adjusting mechanism for an optical system.
2. Description of the Related Art
FIG. 7 illustrates a conventional compact image pickup apparatus.
A barrel 21 has an outer threaded cylindrical surface 21a and a lens holder 23 has an inner threaded cylindrical surface 23a. The barrel 21 is threaded into the lens holder 23 such that the outer threaded cylindrical surface 21a fittingly engages the inner threaded cylindrical surface 23a. The barrel 21 holds a lens 20 mounted therein and has a rear aperture 22 disposed behind the lens 20. The lens holder 23 holds a substrate 26 at a bottom thereof. An image pickup element 25 is mounted on the substrate 26 and has an image region 25a fabricate on a surface thereof. The image region 25a is electrically connected via bonding wires 25b to leads 25c mounted on the substrate 26.
With the aforementioned image pickup apparatus, variations of focussing performance may be encountered during assembly operations. An error in the distance between the lens 20 and the image pickup element 25 determines how precisely the image can be focused on the image pickup element 25. Factors that cause errors in the distance between the lens 20 and the image pickup element 25 include: (1) assembly errors between the lens 20 and the barrel 21, (2) errors in back focus (referred to Bf) due to dimensional errors of the lens 20, (3) dimensional errors of the barrel 21, thickness errors of the infrared filter 24, (4) dimensional errors of the lens holder 23, (5) positional errors of the image region 25a in a direction shown by arrow Z, and (6) positional errors between the image pickup element 25 and substrate 26.
The barrel 21 is fitted into the lens holder 23 by screwing the threaded surface 21a into the threaded surface 23a. Rotating the barrel 21 relative to the lens holder 23 allows the barrel 21 to move relative to the lens holder 23 in the direction shown by arrow Z. The rotation of the barrel 21 allows adjustment of the distance between the lens 20 and the image region 25a, thereby accommodating all errors encountered during manufacture to precisely focus an image on the image region 25a. This conventional image pickup apparatus requires many components. Moreover, the image pickup apparatus suffers from the problem that individual adjustment of focusing is required after the barrel 21 has been assembled to the lens holder 23. Thus, the apparatus does not lend itself to mass production.
FIG. 8 illustrates an example of another conventional image pickup apparatus disclosed in Laid-open Japanese Patent (KOKAI) No. 9-232548.
The construction of this image pickup apparatus provides improved mounting accuracy of the respective structural elements, thereby eliminating the need for adjustment of focusing.
Referring to FIG. 8, a support member 32 is configured and dimensioned to define mounting positions at which the respective structural elements are accurately positioned relative to one another. A position 32b is formed to accurately mount a lens 33 and a mounting portion 32c is formed to accurately position an image pickup element 35. The support member 32 is formed with a recess in its bottom in which an adhesive 37 is introduced, thereby preventing the image pickup element 35 from being raised by the adhesive 37. The support member 32 having mounting positions precisely defined therein not only eliminates a mechanism for adjusting focus but also serves to play a role of the barrel 21 and lens holder 23 of FIG. 7, thereby reducing the number of structural components.
A stop 30 has an entrance pupil 30a formed therein. The stop 30 is accurately positioned with the aid of a mounting position 32a. Reference 35a denotes an image region and reference 35b denotes a bonding wire.
A lead 36 and the support member 32 are preferably formed in one-piece construction. The support member 36 is usually formed of, for example, acrylic, polycarbonate, ABS (acrylonitrile-butadiene-styrene copolymer), PBT (polybutylene terephthalate), or a synthetic resin. Members such as the support member 32 and lead 36 that have extremely different physical properties are difficult to form in one-piece construction. Therefore, the support member 32 is often divided into a two-piece assembly; an upper portion higher above the lead 36 and a lower portion below the leand 36.
FIG. 9 illustrates factors that cause assembly errors, which in turn affect the focusing performance of an image pickup apparatus of the aforementioned construction.
The factors will be described with respect to a case where the support member 32 is a two-piece structure having an upper portion higher above the lead 36 and a lower portion below the lead 36. An error ΔA of the back focus is an error that results from an error of a radius of curvature of the lens 33. When the compact size of an image pickup apparatus is of prime importance, the image pickup element 35 is not usually placed in, for example, a ceramic container and is used in chip form. Thus, the thickness of the wafer of the image pickup element 35 has an error ΔC. The support member 32 has a dimensional error ΔD. Thereis an error ΔE between the image pickup element 35 and the mounting portion 32c. The layer of adhesive between the lens 33 and the support member 32 has a thickness error ΔF. If excessive adhesive 37 is not introduced into the recess, the image pickup element 35 is not raised so that the error ΔE becomes zero. When the upper portion of the support member 32 is connected to the lower portion by means of the adhesive, the adhesive will have a thickness error ΔG. The errors ΔA to ΔD and ΔF and ΔG affect a maximum focus error of the image pickup apparatus. For the image pickup apparatus of the aforementioned construction not to need adjustment of focusing, the sum ΔT=ΔA+ΔC+ΔD+ΔF+ΔG should be smaller than an acceptable depth of focus Δδ. Thus, the errors ΔA, ΔC, ΔD, ΔF, and ΔG need to be closely controlled, requiring highly dimensional accuracy and assembly accuracy.
The conventional image pickup apparatus of the aforementioned configurations require individual focus adjustment during manufacture of image pickup apparatus, being inefficient in mass production.
The need for focus adjustment requires more number of structural components. In order to provide a focus-adjustment free apparatus, the structural elements should have high levels of dimensional accuracy and assembly accuracy.
FIG. 10 illustrates still another conventional apparatus disclosed in Laid-open Japanese Patent (KOKAI) No. 9-121040. This apparatus is free from focus adjustment. A lens 40 brings light rays from a subject into focus on an image pickup element 44 supported on a substrate 46. The lens 40 and a lens-mounting member 41 are formed in one-piece construction. The lens-mounting member 41 includes legs 42 and beveled positioning surfaces 43. The legs 42 are bonded to the substrate 46 by a UV-curing resin. The beveled positioning surfaces 43 are employed to position the lens 40 relative to the image region of the image pickup element 44 such that the optical axis of the lens 40 passes through the center of the image region. However, the beveled surface is apt to fail to align the optical axis of the lens 40 accurately normal to the surface of the image region, i.e., the optical axis may be at an angle with the line normal to the surface by an angle θ as shown in FIG. 11. In order to solve this problem, a fine adjustment mechanism or a special jig is required when the lens-mounting member 41 is fixedly mounted.
With the conventional image pickup apparatus of FIGS. 10 and 11, the lens 40 and the mechanical supporting structure that support the lens 40 are formed in one-piece construction. This one-piece construction eliminates mounting errors between the lens 40 of the optical system and the lens-mounting member 41 and legs 42. In order to form the lens 40, legs 42, and beveled positioning surfaces 43 in one-piece construction, these structural elements must be molded. The lens 40 that focuses an image on the surface of the image region should be made of a transparent material and the other parts should be made of a material that can block light other than image light. Without blocking unwanted light, optical noise will enter the image formed on the image region of the image pickup element 44. Thus, portions other than the lens 40 should be painted black at a later stage of manufacture.
Alternatively, two types of material may be used: a transparent material such as acrylic PMMA for the lens 40 and a black material for other parts. However, forming an optical system by a two-color molding suffers from a serious technical difficulty because the radius of curvature of the lens 40 requires to be very accurately controlled. Thus, molding the optical system from materials of different colors does not lend itself to mass production.
Further, the construction where the lens and lens-mounting member are formed in one-piece construction does not lend itself to mass production.
The construction where the optical holder abuts a part of the image pickup element suffers from the problem that there are limitations on the position at which the substrate is mounted.
The aforementioned conventional apparatus suffer from the inherent problem that the circuit board is disposed under the image pickup element and therefore the thickness of the circuit board adds to the overall size of the image pickup apparatus.